Populations of Drosophila melanogaster can learn - they specifically avoid odors previously associated with electric shock. Single-gene mutants have been isolated which show no learning in this test. Comparing mutant and normal flies should ultimately reveal physiological, anatomical, or biochemical alterations responsible for the block in learning. Behavioral assays are necessary to determine which mutants affect learning per se and which affect sensory or accessory processes. Tests for olfactory capability, learning tests using other sensory modalities, and a procedure to measure learning in individual flies have been developed and are being applied to the mutants. Three of the mutants, selected for deficiency in operant learning, are apparently also affected in more primitive forms of behavioral plasticity. Their capacity for sensitization and habituation is being compared with normal flies. Genetic mapping and complementation studies of the mutants are in progress. Mosaic techniques will be used in conjunction with the single-fly learning assay mentioned above to find the anatomical focus affected by the learning mutations. Several of the mutants have morphological and developmental abnormalities in addition to deficits in learning. These are being investigated, to try to find specific anatomical or biochemical lesions caused by the mutations. Cold anesthesia has been used to show that flies, like higher organisms, have short- and long-term memory phases. Memory storage can be dissociated from learning and studied using mutations. A mutant, amnesiac, has been isolated which learns normally but forgets rapidly. It should be possible to decide which memory phase is affected and to isolate other memory mutants.